Residential gas-fired water heaters commonly comprise a storage tank in which hot water available for use is stored, and a natural-draft, naturally-aspirated burner which heats the stored water by causing products of combustion to flow over surfaces of the storage vessel. Commonly, the storage tank is a cylindrical shape, is made of "glass-lined" steel, and has a centrally located flue through which products of combustion flow. The flue usually contains baffles and serves as a heat exchanger to transmit heat from the combustion products to the water stored within the vessel. While such water heaters can be simply and economically produced, they are subject to corrosion and, for reasons discussed below, are inefficient.
The combustion assembly commonly used in residential storage type water heaters is of the type in which natural gas initially at line pressure or less aspirates and premixes with an amount of air that is insufficient for complete combustion. Such systems rely upon "secondary air" in the combustion chamber for the additional air necessary to complete combustion. Because there are zones adjacent the burner where gas/air mixtures range from overly rich to overly lean, there is a formation of oxides of nitrogen, an undesirable pollutant. Additionally, because the burn-out of carbon monoxide is limited by the rate of mixing of secondary air, carbon monoxide is not rapidly eliminated. For this reason, large combustion chambers are required for burn-up of carbon monoxide.
Conventional center-flue water heaters which employ secondary air for combustion rely upon the buoyancy of the heated air in the central flue to create a draft which induces the flow of secondary air into the combustion chamber. During the off-cycle, the temperature of the air in the central flue is still sufficiently elevated with respect to normal ambient temperatures that a significant "stack effect" is created. This stack effect produces sufficient draft to produce secondary air flows during the off-cycle which are of similar magnitude to the flow of secondary air during the burner on-cycle. This unwanted convection of secondary air increases the rate of heat loss from the stored water through the walls of the central flue.
In efforts to reduce the production of pollutants and to reduce the off-cycle heat losses from the storage tank, power burners have been suggested. See, for example, U.S. Pat. Nos. 3,823,704 to Daugirda et al. and 3,854,454 to Lazaridis. In such systems, an electric blower is used to draw combustion air into the assembly and to provide a near stoichiometric mix of fuel gas and air upstream of the burner. As a result, there are no overly rich or overly lean zones adjacent the burner to promote the formation of oxides of nitrogen, and the burnout of carbon monoxide is not limited by the mixing of secondary air. Further, because there is no requirement for secondary air, the combustin chamber can be substantially closed but for the burner and exhaust flue so that off-cycle convection through the combustion chamber and in heat exchange relationship with the storage tank is minimized. A major fault of such systems is that the blower used in the forced draft and mixing is costly, requires costly electrical power, and has a limited life.
In the Lazaridis patent, the problems of corrosion of the storage tank are avoided by the use of a plastic lined tank. Also, a center flue is not utilized so that there is little heating of the air in the flue during the off-cycle and the undesired resultant convection through the flue is minimized. The use of a plastic lined storage tank without a center flue does present problems of heat exchange between the combustion products and stored water during the on-cycle. Lazaridis overcomes that problem by use of a heat pipe which leads from the combustion chamber to the interior of the storage tank. Unfortunately, the heat pipe results in a further increase in the cost of the heater system, and the life of the heat pipe, which must remain hermetically sealed, is limited.
Daugirda et al. provide for heat exchange to the water and the storage tank by means of a water walled combustion chamber. But water is driven through the tubes in that chamber by a circulator. Like the blower used for premixing of the gas and air, the circulator is costly, requires electrical power, and has a limited life.
An object of this invention is to provide a storage-type water heater, particularly when utilizing a plastic lined storage vessel, which has high recovery efficiency with low standby losses and which minimizes the formation of noxious pollutants. A further object of the invention is to provide such a water heater which can be manufactured at little additional cost over conventional center-flue water heaters and which does not utilize any electrically powered motors.